1. Field of the Invention.
This invention relates to an apparatus and process for imagewise exposing an infrared sensitive layer and, more specifically, for imagewise ablating an infrared radiation sensitive layer of a flexographic printing element for use in making a flexographic printing plate.
2. Description of Related Art.
Flexographic printing plates are well known for use in letterpress printing, particularly on surfaces which are soft and easily deformable, such as packaging materials, e.g., cardboard, plastic films, etc. Flexographic printing plates can be prepared from photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427,749. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable solution removes the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
Imagewise exposure of a photosensitive element requires the use of a phototool which is a mask having clear and opaque areas covering the photopolymerizable layer. The phototool prevents exposure and polymerization in the opaque areas. The phototool allows exposure to radiation in the clear areas so that these areas polymerize and remain on the support after the development step. The phototool is usually a photographic negative of the desired printing image. If corrections are needed in the final image a new negative must be made. This is a time-consuming process. In addition, the phototool may change slightly in dimension due to changes in temperature and humidity. Thus, the same phototool, when used at different times or in different environments, may give different results and could cause registration problems.
Thus, it would be desirable to eliminate the phototool by directly recording information on a photosensitive element, e.g., by means of a laser beam. The image to be developed could be translated into digital information and the digital information used to place the laser for imaging. The digital information could even be transmitted from a distant location. Corrections could be made easily and quickly by adjusting the digitized image. In addition, the digitized image could be either positive or negative, eliminating the need to have both positive-working and negative-working photosensitive materials, or positive and negative phototools. This saves storage space and, thus, reduces cost. Another advantage is that registration can be precisely controlled by machine during the imaging step. Digitized imaging without a phototool is particularly well-suited for making seamless, continuous printing forms.
In general, it has not been very practical to use lasers to image the elements which are used to prepare flexographic printing plates. The elements have low photosensitivity and require long exposure times even with high powered lasers. In addition, most of the photopolymerizable materials used in these elements have their greatest sensitivity in the ultraviolet range. While UV lasers are known, economical and reliable UV lasers with high power are generally not available. However, non-UV lasers are available which are relatively inexpensive, and which have a useful power output and which can be utilized to form a mask image on top of flexographic printing elements.
In view of these facts, a photosensitive printing element 10 has recently been developed for use in preparing flexographic print plates. Referring from bottom to top in FIG. 1, the photosensitive printing element comprises, in order, a support or cushion layer 12, at least one photopolymerizable layer 14 which is substantially insensitive to infrared radiation, one or more optional barrier layer 16, and at least an infrared radiation sensitive layer 18. A removable protective coversheet 22 with an optional release layer 20 cover and protect the outer infrared radiation sensitive layer 18. See, for instance, U.S. Pat. No. 5,262,275 and U.S. patent application Ser. Nos. 08/130,610 and 08/341,731. Also see U.S. patent applications IM-1002 and IM-0971, now U.S. Pat. No. 5,506,086 assigned to E. I. du Pont de Nemours and Company and filed on the same day as this application. The process of using these photosensitive printing elements involves (1) removing any protective coversheet 22 and the release layer 20, if present; (2) imagewise ablating the infrared-sensitive layer 18 to form a mask; (3) overall exposing the photosensitive element 10 to non-infrared actinic radiation through the mask; and (4) treating the product of step (3) with at least one developer solution to remove (i) the infrared-sensitive material which was not removed during step (2), (ii) at least the areas of any barrier layer 16, if present, which were not exposed to non-infrared actinic radiation, and (iii) the areas of the photopolymerizable layer 14 which were not exposed to non-infrared actinic radiation. The treating step (4) produces a flexographic printing plate with a relief surface or image which is inked and used for flexographic printing of typically multiple copies of the inked portions of the relief surface or image.
Conventional laser engraving systems are available for directly engraving relief images in materials for directly producing relief printing surfaces. Typical laser engravers use CO.sub.2 lasers which emit a highly multi-mode beam having a wavelength of about 10.6 microns which are very powerful lasers that would burn through or vaporize most polymers including polymers in conventional flexographic printing elements and the entire elements disclosed in U.S. Pat. No. 5,262,275, U.S. patent application Ser. Nos. 08/130,610 and 08/341,731, and U.S. patent applications IM-1002 and IM-0971, now U.S. Pat. No. 5,500,086. Such engravers typically have a minimum resolution or beam diameter of about 40 microns which is much too large for creating a high resolution flexographic printing mask. The maximum modulation rate of a CO.sub.2 laser is about 20 kHz which causes the maximum engraving speed to be too slow for a commercially viable system for forming high resolution mask layers on flexographic printing elements.
The Grapholas.RTM. system made and sold by Baasel-Scheel Lasergraphics, Gmbh, of Itzehoe, Germany, is for directly engraving relief images in a layer for directly producing relief printing surfaces. The Grapholas.RTM. system comprises:
a support assembly including: PA1 a CO.sub.2 laser assembly for producing an image modulated beam; PA1 an optical assembly including: PA1 a modulated beam delivery carriage assembly including: PA1 a computer assembly for (i) receiving, generating and/or storing the image data representing an image to be engraved in the layer, and (ii) selecting exposure parameters from a set consisting of the cylindrical surface speed, the support advance rate, image placement coordinates on the layer, focusing position, and amplitude modulation level; and PA1 an electronic control assembly for receiving the image data and the exposure parameters from the computer assembly and processing the image data and the exposure parameters to control the support assembly, the laser assembly, the optical assembly, and the linear transport mechanism to engrave the image in a spiral fashion in the layer. PA1 a support assembly including: PA1 a laser assembly for producing an image modulated beam, the laser assembly comprising: PA1 a stationary (i.e., fixed) optical assembly including: PA1 a linear transport mechanism for transporting the support, the drum and the drum motor assembly along a line to scan the image modulated beam along the layer on the cylindrical surface at a carriage advance rate of about an engraving width of the modulated beam on the layer on the cylindrical surface; PA1 a computer assembly for (i) receiving, generating and/or storing the image data representing an image to be engraved in the layer, and (ii) selecting exposure parameters from a set consisting of the cylindrical surface speed, the support advance rate, image placement coordinates on the layer, focusing position, and amplitude modulation level; and PA1 an electronic control assembly for receiving the image data and the exposure parameters from the computer assembly and processing the image data and the exposure parameters to control the support assembly, the laser assembly, the optical assembly, and the linear transport mechanism to engrave the image in a spiral fashion in the layer. PA1 a support assembly including: PA1 a laser assembly for producing an image modulated beam; PA1 an optical assembly including: PA1 a modulated beam delivery carriage assembly including: PA1 a computer assembly for (i) receiving, generating and/or storing image data representing an image to be exposed on the layer, and (ii) selecting exposure parameters from a set consisting of the cylindrical surface speed, the carriage advance rate, image placement coordinates on the layer, focusing position, and amplitude modulation level; and PA1 an electronic control assembly for receiving the image data and the exposure parameters from the computer assembly and processing the image data and the exposure parameters to control the support assembly, the laser assembly, the optical assembly, and the modulated beam delivery carriage assembly to expose the image in a spiral fashion in the layer; and wherein the improvement comprises: PA1 (1) providing a photosensitive printing element comprising in the order listed: PA1 (2) imagewise ablating layer (d) with an infrared laser radiation beam having a peak power density of 0.1 megaWatts/cm.sup.2 to 17 megaWatts/cm.sup.2 and adapted to provide an energy density of 0.5 Joules/cm.sup.2 to 5 Joules/cm.sup.2 from a laser ablating apparatus to form a mask, the apparatus comprising a support assembly, a laser assembly, an optical assembly, a modulated beam delivery carriage assembly, a computer assembly, and an electronic control assembly; PA1 (3) overall exposing the photosensitive element to non-infrared radiation actinic radiation through the mask; and
a support, PA2 a drum rotatably mounted on the support, the drum having an outer cylindrical surface, the layer adapted to be mounted on the cylindrical surface, and PA2 a drum motor assembly mounted on the support, the drum motor assembly for rotating the drum with a maximum speed of about 200 rpm; PA2 a lens for focusing the directed beam having a minimum resolution of about 40 microns at the layer on the cylindrical surface, and PA2 a focusing motor assembly for moving the lens with respect to the layer on the cylindrical surface to focus the modulated beam such that the focused modulated beam is adapted to engrave the layer; PA2 a carriage for supporting the optical assembly, PA2 a linear track along which the carriage is adapted to move, the track parallel to the longitudinal axis, and PA2 a translator motor assembly for transporting the carriage along the track to scan the image modulated beam along the layer on the cylindrical surface at a carriage advance rate of about an engraving width of the modulated beam on the layer on the cylindrical surface; PA2 a support, PA2 a drum rotatably mounted on the support, the drum having an outer cylindrical surface, the layer adapted to be mounted on the cylindrical surface, and PA2 a drum motor assembly mounted on the support, the drum motor assembly for rotating the drum with a maximum speed of about 106 rpm; PA2 a Nd:YAG laser for emitting an output infrared radiation beam, PA2 a laser power supply for energizing the laser, and PA2 a Q-switch for image modulating the output infrared radiation beam; PA2 a lens for focusing the directed beam at the layer on the cylindrical surface, and PA2 a focusing motor assembly for moving the lens with respect to the layer on the cylindrical surface to focus the modulated beam such that the focused modulated beam is adapted to engrave the layer; PA2 a rotatable cylindrical surface having a longitudinal axis, the layer adapted to be mounted on the cylindrical surface, and PA2 a motor assembly for rotating the cylindrical surface; PA2 a lens for focusing the directed beam at the layer on the cylindrical surface, and PA2 a focusing motor assembly for moving the lens with respect to the layer on the cylindrical surface to focus the modulated beam such that the focused modulated beam is adapted to engrave the layer; PA2 a carriage for supporting the optical assembly, PA2 a linear track along which the carriage is adapted to move, the track parallel to the longitudinal axis, and PA2 a translator motor assembly for transporting the carriage along the track to scan the image modulated beam along the layer on the cylindrical surface at a carriage advance rate of about an exposing width of the modulated beam on the layer on the cylindrical surface; PA2 the focused modulated beam comprises infrared radiation having a peak power density of 0.1 megaWatts/cm.sup.2 to 17 megaWatts/cm.sup.2 and is adapted to provide an energy density of 0.5 Joules/cm.sup.2 to 5 Joules/cm.sup.2 on the layer. PA2 (a) a support layer, PA2 (b) at least a photopolymerizable layer comprising an elastomeric binder, at least a monomer and at least an initiator having sensitivity to non-infrared actinic radiation, said photopolymerizable layer being soluble, swellable or dispersible in a developer solution; PA2 (c) optionally at least a barrier layer which is substantially transparent to non-infrared actinic radiation; and PA2 (d) at least a layer of infrared radiation sensitive material which is substantially opaque to non-infrared actinic radiation;
Another engraving system previously made and sold by Baasel Lasertechnik, Gmbh called the Grapholas.RTM. Compact system was also for directly engraving relief images in a layer for directly producing relief printing surfaces. The Grapholas.RTM. Compact system comprised:
Since the structure and properties of the photosensitive printing elements disclosed in U.S. Pat. No. 5,262,275, U.S. patent application Ser. Nos. 08/130,610 and 08/341,731, and U.S. patent applications Ser. No. IM-1002 and IM-0971, now U.S. Pat. No. 5,506,086 are different than other materials that have been engraved by conventional engraving machines, such machines are not capable of forming a mask from the infrared radiation sensitive layer of the photosensitive printing elements disclosed in U.S. Pat. No. 5,262,275, U.S. patent application Ser. Nos. 08/130,610 and 08/341,731, and U.S. patent applications Ser. No. IM-1002 and IM-0971, now U.S. Pat. No. 5,506,086 in a commercially viable manner.
In fact, since these unique photosensitive printing elements were only recently developed, no apparatus is commercially available which is capable of ablating the infrared radiation sensitive layer of these photosensitive printing elements to form a suitable mask for use in preparing flexographic printing plates with desired productivity and image quality.